Birches rank second only to oaks among allergenic trees, according to Pollen.com's Pollen Library. They form fast-growing forests that quickly colonize new habitat, becoming increasingly allergenic as they multiply.
Habitat: Europe, Asia and North America (especially east of the Rocky Mountains)
Description: Tall, narrow trees growing 60 to 100 feet high, often with scaly, papery bark
Allergenicity: Moderate to severe
Allergy season: Spring
Tips: Birches are at their worst in large numbers, but if you're allergic, actual birch trees may not be your only problem: Proteins in many raw fruits and vegetables resemble birch pollen enough to trigger allergies in some people. These include almonds, apples, cherries, peaches and walnuts, but fortunately the proteins can be denatured by cooking.